I'm guessing it won't be till the oft-hypothesized re-jiggering of how much territories cost, unless some rich Brony truly has a passion project. Which I am here for, if any such person is lurking.

Good question -- there must be some sufficient size where it can be viable on a general-interest site, or a site with another interest that is not Bronies. I've been pleasantly surprised by the activity my territory ~mostly_harmless gets, which is not as fringe as MLP, but which is also not even about any particular topic, more a way of being.

And yet people show up! So maybe whatever the carrying-size required for Bronies is, we're at it?